1. Field of the Invention
This invention relates to a method for removing volatile metals from slag using an acid wash.
2. Description of the Prior Art
Ash-containing feedstocks such as petroleum coke, coal, residual fuel oils or other contaminated hydrocarbonaceous materials that undergo partial oxidation in a partial oxidation reactor frequently produce a slag byproduct.
A typical charge to a partial oxidation reactor includes the feedstock, a free-oxygen containing gas and any other materials that may enter the burner located in the reactor inlet.
As noted herein, the partial oxidation reactor can also be referred to as a "partial oxidation gasifier reactor", "gasification reactor", or simply a "reactor" or "gasifier," and these terms are often used interchangeably.
Partial oxidation reactors are well known in the art, as are the partial oxidation reaction conditions. See, for example, U.S. Pat. Nos. 4,328,006 and 4,328,008, both to Muenger, et al., U.S. Pat. No. 2,928,460 to Eastman, et al., U.S. Pat. No. 4,872,886 to Henley et al., U.S. Pat. No. 4,760,667 to Eckstein et al., U.S. Pat. No. 4,823,741 to Davis et al., U.S. Pat. No. 4,889,540 to Segerstrom et al., U.S. Pat. Nos. 4,959,080 and 4,979,964, both to Sternling, and U.S. Pat. No. 5,281,243 to Leininger.
The partial oxidation reaction is conducted under reaction conditions that are sufficient to convert a desired amount of ash-containing feedstock to synthesis gas or "syngas". Non-gaseous byproducts include slag and other materials, such as char, fine carbon particles, and inorganic ash. The slag and inorganic ash is frequently composed of metals such as iron, sodium, potassium, aluminum, calcium, silicon, and the oxides and sulfides of these metals, much of which is entrained in the syngas product stream.
Reaction temperatures typically range from about 900.degree. C. to about 2,000.degree. C., preferably from about 1,200.degree. C. to about 1,500.degree. C. Pressures typically range from about 1 to about 250 atmospheres, preferably from about 15 to about 200 atmospheres. The average space velocity in the partial oxidation reaction zone generally ranges from about 0.5 to 0.05 seconds.sup.-1, and normally from about 0.3 to about 0.1 seconds.sup.-1.
The syngas product composition will vary depending upon the composition of the feedstock and the reaction conditions and generally includes CO, H.sub.2, steam, CO.sub.2, H.sub.2 S, COS, CH.sub.4, NH.sub.3, N.sub.2, and, if present in the feed to the partial oxidation reactor at high enough concentrations, less readily oxidizable volatile metals, such as lead, zinc, and cadmium.
Slag is essentially fused mineral matter, such as ash, and can also include other carbonaceous materials, such as soot or char. Slag materials also include the oxides and sulfides of transition metals such as vanadium, molybdenum, chromium, tungsten, manganese, and palladium, which can be recovered as valuable byproducts of the slag.
Most inorganic components of gasifier feeds can form slag, which when cooled is a vitreous solid composed of these constituents and mixtures of these constituents and their oxides and sulfides. Such slag-forming elements, also referred to as "slagging elements", include transition metals, such as vanadium, iron, nickel, tantalum, tungsten, chromium, manganese, zinc, cadmium, molybdenum, copper, cobalt, platinum, palladium; alkali and alkaline earth metals, such as sodium, potassium, magnesium, calcium, strontium, or barium; and others including aluminum, silicon, phosphorus, germanium, gallium, and the like. The amount of slagging elements in the feedstock generally varies from about 0.01 to about 50 weight %.
Under the conditions in the gasification reactor, depending on the chemical form and physical characteristics, some metals are non-volatile and others are volatile. The most common non-volatile metals, or metals that form non-volatile compounds, such as oxides under gasifier conditions, include iron, sodium, potassium, aluminum, calcium and silicon, most of which become incorporated in the vitreous slag.
The term "incorporation" as used herein means the ability of certain elements and/or compounds to become chemically or physically bound within the slag, thereby becoming resistant to leaching from the slag.
For purposes of this invention, the term "slag" is also used to collectively refer to inorganic ash and other particulate materials which occur as a result of gasification reactions.
The molten slag produced in partial oxidation reactors can be removed from the process in two ways:
1) In the quench mode of operation, the products of the gasification reactions are cooled by being passed through a pool of water in a quench chamber immediately below the gasifier. Slag is cooled and collects in this quench chamber, from which it and other particulate materials that accumulate in the quench chamber can be discharged from the gasification process by use of a lockhopper or other suitable means. The syngas exiting the quench chamber is passed through an aqueous scrubber for further removal of particulates before further processing. PA1 2) In some applications it is beneficial to recover heat from the hot partial oxidation products. This can be done using a waste heat boiler in which the high temperature syngas and slag are cooled indirectly with water to produce high pressure steam. In this mode of operation, the slag can pass into a pool of water in the bottom of the waste heat boiler where it is cooled and collected, while the gases are removed without water quenching for further cooling and water scrubbing for particulate removal as in the quench mode.